Display devices are known in graphic display and imaging art. Examples of such devices are liquid crystal displays (LCD) and electroluminiscent devices, such as LED (light emitting diode) based devices, organic light emitting diode (OLED) devices and polymer light emitting diode (polyLED) devices.
An electroluminiscent device is a device comprising electroluminiscent material capable of emitting light when a current is passed through it, the current being supplied by means of electrodes. Electroluminiscent devices of the diode type preferentially pass current in one direction and generally comprise electroluminiscent material disposed between an anode and a cathode.
Display devices are often sensitive to oxygen and moisture etc and, consequently, may degrade when exposed to the atmosphere. Exposure to oxygen and/or moisture, particularly in the presence of light, may lead to photo-oxidative degradation of a polymer material used.
Furthermore, oxidation at the cathode/polymer interface is one of the first problems arising in connection to oxygen and/or moisture diffusion in devices such as OLEDs or polyLEDs. Such reactions will significantly reduce the performance of the light emitting properties of the device.
Moreover, other materials used in the display device may also degrade when exposed to the atmosphere. Therefore, there is a need for sealing the device.
Efforts have been made by evaporating metal films to seal display devices. However, these films often contain pinholes. Therefore, these films need to be relatively thick, which results in poor light transmission. Many of the known processes for achieving a more or less proof encapsulation of the devices require temperatures above 300° C. during the sealing process. Most polymer-based devices are not compatible with such high temperatures.
Furthermore, display devices, and in particular the actual diode structures of a polyLED/OLED display, are sensitive to mechanical forces applied to the devices. Scratching the surface of a sealed display device could lead to the breaking of the surface. The surface at structures in the device, such as negative resists and other protrusions, are especially sensitive. Breaking of the surface would affect the display in the sense of loss of function in parts of the display device. It would also mean that a physical hole appear in the sealing of the surface, through which moisture and oxygen can permeate and reduce the lifetime of the device. Thus there is a need for encapsulation of the sealed device.
U.S. Pat. No. 6,413,645 discloses a barrier assembly comprising a barrier layer and a polymer layer formed on top of the barrier layer. The assembly may be applied on top of a display device with low temperature methods, thus covering and protecting the device from water and oxygen. The barrier assembly consists of at least one layer of a barrier material, such as metal oxides, metal nitrides and metal carbides etc, that acts as water/oxygen barriers. The polymer layer has no intrinsic barrier function, but works as a planarisation layer.
However, the barrier assembly disclosed in U.S. Pat. No. 6,413,645 is vulnerable to mechanical forces. If a mechanical force is applied to the device, the barrier assembly would be damaged, and the water/oxygen permeability would be drastically impaired. In order to strengthen the assembly, the deposition of additional layers, such as protective polymer layers and scratch resistant layers, on the barrier assembly is proposed. However, these extra layers introduces additional steps into the manufacturing method, which thus becomes complicated and expensive, and the total barrier assembly with the extra protective layers becomes relatively thick.